Dr. John Snow is regarded as one of the founding father of modern
epidemiology.
London suffered a series of cholera outbreaks during the mid-19th century,
and Snow theorized that cholera reproduced in the human body and was
spread through contaminated water.
The conventional wisdom at the time
was that diseases were spread by "miasma" in the air.

London's water supply system consisted of shallow public
wells where people could pump their own water to carry home,
and about a dozen water utilities that drew water from
the Thames to supply a jumble of water lines to more upscale
houses.
London's sewage system was even more ad hoc: privies emptied into
cesspools or cellars more often than directly into sewer pipes.
The pervasive stench of animal and human feces combined with rotting
garbage made the miasma theory of disease seem very plausible:
Disease was more prevalent in lower-class neighborhoods because
they stank more, and because the moral depravity of the lower classes
made them more vulnerable to disease.

The September 1854 cholera outbreak was centered in the Soho district,
close to Snow's house.
Snow mapped the 13 public wells and all the known cholera deaths around
Soho, and
noted the spatial clustering of cases around one particular water
pump on Broad Street.
He examined
water samples from various wells under a microscope, and confirmed the
presence of an
unknown bacterium in the Broad Street samples.
Despite strong scepticism from the local authorities, he removed
the
handle from the Broad Street pump and halted the outbreak.

Snow subsequently published his map with a discussion of his analysis
of it. An
image of a medium-resolution
downloadable scan of Snow's map available from Wikipedia is shown
below.
This map shows the locations of the 13 public water sources in and
around Soho, and
shows cholera deaths by home address, marked as black bars stacked
perpendicular to the streets.

Although the large workhouse just north
of Broad Street housed over 500 paupers, it suffered very few cholera
deaths because it had its own well (not shown on the map).
Likewise, The workers at the brewery one block east
of the Broad Street were allowed all the beer they could drink
on the job, and didn't use the well, and none of them contracted cholera.
Many of the deaths further away from the Broad Street pump
were people who walked to work or market on the Broad Street and
drank from that well. Other deaths were attributable to the
superior taste of the water from Broad Street, particularly
compared to the smelly water from the Carnaby Street/Little Marlborough
Street well.